Steelmaking slag (such as converter slag, pretreatment slag, secondary refining slag and electric furnace slag) generated in a steelmaking process is known to contain oxides of, e.g., phosphorus (P), calcium (Ca), iron (Fe), silicon (Si), manganese (Mn), magnesium (Mg), and aluminum (Al). Specifically, calcium exists in steelmaking slag as remaining quicklime (CaO) which is fed during a steelmaking process, free lime precipitated from the quicklime during a solidification period, or as calcium hydroxide (Ca(OH)2) or calcium carbonate (CaCO3) each generated from the free lime reacting with water vapor or carbon dioxide in the air.
Steelmaking slag is used in various applications, such as cement materials, road base materials for roads, construction materials and fertilizers (see NPLs 1 to 3). However, free lime contained in steelmaking slag should be handled with care since it inflates products or is eluted as high alkaline water upon contact with water (see NPL 1).
Calcium is used in a sintering process of iron making as calcium carbonate. Calcium oxide obtained by calcining calcium is used in a steelmaking process. Calcium hydroxide obtained by adding water to calcium oxide is used as a neutralizer for, e.g., acids in a draining process. Therefore, when calcium can be recovered from steelmaking slag generated in an iron-making process, calcium can be reused to reduce iron-making cost.
Accordingly, attempts to recover calcium from steelmaking slag are being made (see PTLs 1 to 3).
PTL 1 describes a method in which carbon dioxide is blown into an aqueous solution having calcium eluted from converter slag to allow calcium carbonate to settle out and recover the same. During this procedure, the lower limit of the pH is maintained at about 10 for suppressing the generation of calcium hydrogen carbonate which is highly soluble in water. Although PTL 1 does not describe a specific way for maintaining the pH at 10 or more, the inventors consider that the pH is maintained at 10 or more by adjusting the amount of blown carbon dioxide during the performance of the method.
PTL 2 describes a method in which fractured steelmaking slag is separated into an iron-condensed phase and a phosphorus-condensed phase, a calcium component in the phosphorus-condensed phase is dissolved as calcium hydrogen carbonate in rinse water containing carbon dioxide dissolved therein, and then the rinse water is heated to 50 to 60° C. to allow calcium hydrogen carbonate therein to settle out as calcium carbonate.
PTL 3 describes a method in which calcium compounds are eluted from steelmaking slag by multiple process. PTL 3 describes the recovery method as capable of preferentially eluting 2CaO/SiO2 phase and phosphorus in a state of solid solution therein by immersing steelmaking slag (pretreatment slag) several times in water containing carbon dioxide blown therein.